The spraying method is a surface treatment method wherein fine powder of metal, ceramic, cermet or the like is fused by using plasma or a combustion energy of a combustible gas and sprayed onto a surface of a substrate to from a sprayed coating.
For example, when metal powder is sprayed in air, fine metal particles of fused state contact with air to produce an oxide film on the surface of the particle. Such a metal sprayed coating has a particle lamination structure of piling fine metal particles having a thin oxide film on their surfaces one upon the other. Therefore, the sprayed coating has problems that bonding force between mutual laminated metal particles constituting the coating is weak and the coating becomes porous to bring about the lowering of the adhesion property to the metal substrate.
In order to solve the above problems, there have hitherto been developed a method of refusing the sprayed coating by heating after the formation of the coating and an alloy suitable for this method or self-fluxing alloy (JIS H8303 Self-Fluxing Alloy Spraying).
The material for the above self-fluxing alloy spraying is a low melting point material consisting essentially of Ni or Co and added with C, Cr, Fe, Mo, Cu, W or the like and further added with Si (1.5.about.5.0 wt %) and B (1.0.about.4.5 wt %). This material lies in a point that the formation of oxide on the sprayed coating is prevented and hard chromium carbide and metal boride are formed to improve the wear resistance. Moreover, it is said that as the above material, it is favorable to use a spraying material of cermet state obtained by mixing the self-fluxing alloy and WC powder if it is intended to more enhance the wear resistance of the sprayed coating.
The conventional techniques for spraying the self-fluxing alloy are mentioned below.
(1) Utilize metallurgical bonding ability through heating-fusion phenomenon (JP-A-6-34041, JP-A-7-226285, etc.).
(2) Attempt the improvement of wear resistance of coating after refusing treatment (JP-A=9-25582, etc.).
(3) Attempt the improvement of resistance to metal fusion and corrosion resistance of self-fluxing sprayed coating after refusing treatment (JP-A-8-158030, JP-A-9-31576, JP-A-9-25582, etc.).
(4) Utilize high adhesion property, heat resistance, resistance to erosion and the like in self-fluxing alloy sprayed coating after refusing treatment to surface coating of a heat conduction tube of a boiler (JP-A-7-278778, JP-A-8-13119, etc.).
(5) Example of self-fluxing alloys other than the self-fluxing alloy defined in JIS H8303 (JP-A-52-99951, etc.).
(6) Decrease production cost by omitting refusing treatment of sprayed coating and exclude degradation of mechanical properties of substrate due to high temperature heating (JP-A-8-225917, etc.).
(7) Conventional technique for refusing treatment of self-fluxing alloy sprayed coating
a. refusing treatment through a laser: JP-B-62-27561
b. refusing treatment utilizing high frequency induction heating: JP-A-7-278778, JP-A-8-253853, etc.
c. control of atmosphere in refusing treatment: JP-A-53-34634 and so on
As mentioned above, the conventional techniques for the self-fluxing alloy sprayed coating are researches and development on not only the improvement of coating properties and enlargement of their application field but also the heating and fusing treatment of the coating, but they are not yet completed to sufficient level and have the following technical problems at the present time.
(1) When the self-fluxing alloy sprayed coating is made to a fused state by heating, a portion of the coating that is easily subjected to heat locally flows to bring about the ununiformity of the coating thickness and drop-off from the surface of the substrate.
(2) When the self-fluxing alloy sprayed coating is subjected to refusing treatment, through-holes in the coating disappear and sprayed particles constituting the coating bond to each other and also metallurgically bond to the substrate. Therefore, the properties of the coating are fairly improved as compared with those before the refusing treatment. However, closed bubbles are existent in the coating, so that when the surface of the sprayed coating is polished, the bubbles are exposed on the surface and hence the smooth surface is not obtained and the application of the coating is restricted.
It is, therefore, an object of the invention to propose a method capable of preventing the fusion drop-off phenomenon of the coating when the self-fluxing alloy sprayed coating is subjected to the refusing treatment.
It is another object of the invention to propose a method capable of preventing the ununiformization of the coating thickness due to the flowing of the coating.
It is the other object of the invention to produce a member coated with a self-fluxing alloy sprayed coating having a smooth surface.